Noise floor:

HF reception, in the range 1.8 - 30 MHz, is often externally noise-limited due
to galactic (sky), atmospheric and/or man-made noise. The curves in Fig.1
illustrate the effective contribution of these noise sources, expressed in dB above thermal
noise.

The Y-axis on the graph is usually specified as Fa, the noise figure
(in dB relative to kToB), and is a measure of the external noise
impinging on the antenna. The Y-axis can also be specified in terms of noise
temperature ta.

In the range 4 - 30 MHz, a good approximation for the minimum external noise is:

Fa = 52 - 23log10f where f =
frequency in MHz.

In the range 0.01 - 4 MHz, this becomes:

Fa = 66 - 47log10f where f =
frequency in MHz.

To ensure that it is always externally noise-limited, an HF receiver system,
located at a quiet radio site and operating at the higher end of the HF range,
should have F < 16 dB. This is equivalent to a -158 dBm/Hz noise floor or a
sensitivity spec. of -113 dBm (0.5 µV) for 10 dB SINAD (equivalent to a
-123 dBm noise floor) in a 3 kHz bandwidth.

As can be seen from Fig.1, this is a limiting situation at HF, and in many
cases, modest overall receiver system sensitivity can be tolerated without
compromising system performance. It should be remarked, though, that additional
receiver sensitivity is still useful to overcome losses in the antenna,
feedline, preselector and so forth.